1. Field of the Invention
This invention relates broadly to methods of coating transparent articles such as glass. More particularly, this invention relates to methods for producing metallic nickel-silver films by chemical replacement reactions.
2. Description of the Prior Art
Transparent articles bearing a transparent metal film have been produced by a variety of well-known electroless or wet chemical coating or plating techniques. These techniques generally involve contacting the article to be coated with suitable solutions to deposit a metal film thereon by reducing the metal from a metal salt in the solution.
Autocatalytic coating techniques are known wherein a reducing agent such as an alkali metal hypophosphite or formaldehyde is present in a single solution with the metal salt. Such a solution is not usually susceptible to rapid reaction until it is placed in contact with a catalytic or sensitized surface. Autocatalytic techniques for the deposition of cobalt are described in U.S. Pat. No. 2,532,283 and 2,532,284 to Brenner and Riddell. Other techniques of this type are described in patents such as U.S. Pat. No. 2,956,900 to Carlson et al, relating to the deposition of nickel.
Exhaustive electroless coating techniques are also known. In these techniques, the reduction reaction proceeds rapidly when the reducing agent and the metal salt are present together in the coating solution. For this reason, such techniques usually require applying separate solutions of these reactants substantially simultaneously to the substrate to be coated. Among these techniques are those wherein a boron-containing reducing agent is employed, such as described in U.S. Pat. No. 3,671,281, to Miller and Cavitt for the deposition of iron, cobalt and nickel.
The reflectance and transmittance characteristics of a transparent article may also be modified by staining techniques whereby metals are introduced into the surface of the substrate. These staining techniques are described in U.S. Pat. No. 2,701,215 to Kroecl and U.S. Pat. No. 3,079,264 to Grego et al. Such techniques are not, however, coating techniques and do not result in a metal film on the surface of the substrate. Nor are such techniques replacement reactions since there is no metal film involved to be replaced.
Replacement reactions for bulk metals using immersion plating baths are known. As described in the METAL FINISHING GUIDEBOOK DIRECTORY for 1965, gold metal is known to replace copper in copper alloys at a temperature of 150.degree.-180.degree.F., using an immersion plating bath containing potassium gold cyanide, sodium cyanide and soda ash. It is also known that gold will replace silver after immersion for 24 hours in a plating bath containing sodium thiosulfate and gold chloride. However, the replacement reactions for bulk metals do not encounter the difficulties which arise when a transparent coated article is the desired product.
The preparation of thin, transparent films on transparent substrates, particularly on large sheets of glass for architectural applications, presents particular problems which are not present in the coating of opaque, especially opaque metal, substrates. In the coating of large, transparent articles for architectural use, it is extremely important that films of uniform thickness, transmittance and reflectivity be produced so that the articles will have a uniform aesthetic appearance. Films of the requisite uniformity have been prepared comprising various metals, including iron, cobalt, nickel, copper and silver, by direct deposition onto the substrate utilizing electroless coating techniques.
Patented electroless coating techniques have been employed with success in producing highly uniform films having an aesthetically pleasing appearance and high reflectivity for energy in the infrared range. Articles coated in accordance with these techniques are used in buildings to provide attractive viewing enclosures which effectively reflect solar energy, thus minimizing the load imposed on air conditioning systems servicing these buildings.
Commercial architectural products such as described above have been produced according to the teachings of U.S. Pat. No. 3,457,138 to Miller, which describes a method of making highly efficient infrared radiation reflective copper-silver films by the direct deposition of a copper film over a directly deposited silver film.
The first reference to replacement reactions in the art of transparent metal films is the copending application of Charles B. Greenberg and Peter P. Harmon, U.S. Ser. No. 384,763, now Pat. No. 3,920,864 which describes the replacement of copper by silver.